The invention relates to a pane with an electrical connection element, an economical and environmentally friendly method for its production, and its use.
The invention further relates to a pane with an electrical connection element for motor vehicles with electrically conductive structures such as, for instance, heating conductors or antenna conductors. The electrically conductive structures are customarily connected to the onboard electrical system via soldered-on electrical connection elements. Due to different coefficients of thermal expansion of the materials used, mechanical stresses occur during production and operation that strain the panes and can cause breakage of the pane.
Lead-containing solders have high ductility that can compensate the mechanical stresses occurring between an electrical connection element and the pane by plastic deformation. However, because of the End of Life Vehicles Directive 2000/53/EC, lead-containing solders must be replaced by leadfree solders within the EC. The Directive is referred to, in summary, by the acronym ELV (End of Life Vehicles). Its objective is, as a result of the massive increase in disposable electronics, to ban extremely problematic components from products. The substances affected are lead, mercury, cadmium, and chromium. This relates, among other things, to the implementation of leadfree soldering materials in electrical applications on glass and the introduction of corresponding replacement products.
The leadfree solder compounds known to date, as disclosed, for example, in EP 2 339 894 A1 and WO 2000058051, are, however, not capable of compensating mechanical stresses to the same extent as lead due to their lower ductility. The customary copper-containing connection elements have, however, a higher coefficient of thermal expansion than glass (CTE(copper)=16.8×10−6/° C.), as a result of which damage to glass occurs upon thermal expansion of the copper. For this reason, connection elements that have a low coefficient of thermal expansion, preferably on the order of magnitude of soda lime glass (8.3×10−6/° C. for 0° C.-320° C.), are preferably used in conjunction with leadfree solder compounds. Such connection elements hardly expand upon heating and compensate the developing stresses.
EP 1 942 703 A2 discloses an electrical connection element on panes of motor vehicles, wherein the difference of the coefficients of thermal expansion of the pane and the electrical connection element is <5×10−6/° C. and the connection element contains predominantly titanium. In order to enable adequate mechanical stability and processability, it is proposed to use an excess of soldering compound. The excess of soldering compound flows out of the intermediate space between the connection element and the electrically conductive structure. The excess of soldering compound causes high mechanical stresses in the glass pane. These mechanical stresses ultimately lead to breakage of the pane. In addition, titanium has poor solderability. This results in poor adhesion of the connection element on the pane. Moreover, the connection element must be connected to the onboard electrical system via an electrically conductive material, for example, copper, possibly by welding. Titanium has poor weldability.
EP 2 408 260 A1 describes the use of iron-nickel or iron-nickel-cobalt alloys such as Kovar or Invar, which have a low coefficient of thermal expansion (CTE). Both Kovar (CTE=5×10−6/° C.) and Invar (CTE up to 0.55×10−6/° C. depending on composition) have a lower CTE than soda lime glass and compensate mechanical stresses. Invar has such a low coefficient of thermal expansion that overcompensation of these mechanical stresses occurs. This results in compressive stresses in the glass or tensile stresses in the alloy which are, however, considered noncritical.
The conventional connection elements made of copper, which are used in conjunction with lead-containing soldering compounds, are unsuitable for soldering with the known leadfree soldering compounds on glass due to their high coefficient of expansion. Connection elements made of iron or titanium do, in fact, have a lower coefficient of expansion and are compatible with leadfree soldering compounds; however, these materials are substantially more difficult to form. Thus, the service life of the tools necessary for the production of the connection elements is reduced, which results in an increase in production costs. Furthermore, with changing materials and shapes of the connection elements, the basic conditions of the soldering procedure have to continually be varied. Different connection elements also have a different mechanical robustness relative to pulling forces. Standardization would thus be desirable to ensure consistent mechanical stability and uniform soldering behavior.